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Spark Plugs breaking tip in Cyl #3

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#1
fotostars

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Ok, I haven't posted in a while but this one is driving me crazy... NA1.8 ...

Every ~3 weekends, I look at my plugs and it's now the 3rd time I find this. Always on Cylinder #3:

 

xl9pag.jpg

 

 

And best of all, the engine is not running rough... Maybe a little down on power but no significant changes.

 

I looked with a bore scope and the piston/chamber looks fine/identical to the others.

 

I tried Autolite as well as NGK, same result.

 

Clues??

 

Richard.


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#2
FTodaro

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What plug  and number are you running?


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#3
fotostars

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What plug  and number are you running?

 

Tried NGK - BKR6E and Autolite Racing - AR3935

gapped at 0.042

 

Kevin @ MCE Racing swears by Autolite so that's what he installed last time we did a dyno run.

 

We may have seen the results of it when for no reason the dyno sent from 121hp to 114hp. We looked at the plugs (NGK) and cylinder #3 had the broken tip. Replaced with new Autolite and came back to ~121-122hp.

He has no idea what could cause this.


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#4
Steve Scheifler

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Detonation can do that, possibly due to a weak injector, but the plug looks rich rather than lean. Could still detonate if there is a hot spot for some reason or the plug leaks at idle making it look rich but does not flow or atomize well at high demand.

Do you know what other SMs make on the same dyno? Maybe your highs are actually a bit low.
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#5
fotostars

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Detonation can do that, possibly due to a weak injector, but the plug looks rich rather than lean. Could still detonate if there is a hot spot for some reason or the plug leaks at idle making it look rich but does not flow or atomize well at high demand.

Do you know what other SMs make on the same dyno? Maybe your highs are actually a bit low.

 

Uhm, all the plugs are black, so rich right?  Anyhow, I guess it's simple enough to take the injectors out and run a quick test on them. I like the idea.


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#6
Dave D.

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Is there possibly a spark plug threaded insert repair in there? I have seen the ones made of stainless not let the plug shed heat into the head due to it's slow heat transfer properties. The plugs would burn out withing a year in a street car. I don't repair stripped plug holes anymore....



#7
fotostars

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Is there possibly a spark plug threaded insert repair in there? I have seen the ones made of stainless not let the plug shed heat into the head due to it's slow heat transfer properties. The plugs would burn out withing a year in a street car. I don't repair stripped plug holes anymore....

 

Haaa. Good thinking on temperature not dissipating... But no repair: the head is still a single block of metal.


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#8
Steve Scheifler

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Yeah, good thought Dave, they could also create a hot spot if the end extends beyond the casting.

If all look that rich, after solving the immediate issue I would look at that next.
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#9
Duane Polsley

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Saw this once, when a guy put anti-seize on his plugs before installing,  I think it keeps the plug from grounding properly.


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#10
Ron Alan

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Definitely have seen that on a repaired head(helicoil). But I think if you have tried different plugs with the same result and it would seem you have fuel(black color)...have you replaced plug wire? Coil? 


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#11
fotostars

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OK, Looked through my boxes of old spark plugs a second time and couldn't find a NGK with a broken tip... So maybe it's the Autolite ?

 

Ron, wires have 8 weekends on them and Coil are unknown... I guess I can always swap wires 2 & 3 at the coil and monitor, see if the issue reproduces and follows the coil... 

I guess I'm always a little careful throwing parts at a problem so I'll start with fresh NGK plugs and monitor at every sessions.

 

Richard.


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#12
SaulSpeedwell

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Piston is hitting it?  Miatas have dangerously close clearance, despite being non-interference.  


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#13
tom1977

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Is there any carbon build up in that cylinder, IE higher compression than the other cylinders?  Have you checked to see if there is even compression between cylinders?  This one has me stumped...



#14
fotostars

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Piston is hitting it?  Miatas have dangerously close clearance, despite being non-interference.  

 

Uhm, I doubt it but who knows... I used a camera to look inside and there is no obvious marking on the piston that would indicate this is the case.

 

Is there any carbon build up in that cylinder, IE higher compression than the other cylinders?  Have you checked to see if there is even compression between cylinders?  This one has me stumped...

 

Some build up on the piston but similar to the other cylinders, the engine has 3 seasons. Haven't checked the compression this past year, I'll do that...


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#15
SaulSpeedwell

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Any update on this? 


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#16
fotostars

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I've replaced the injectors. This was the most possible cause offered to me

So far so good. The thing is: I just don't know how long it really took for it to break, so I'll monitor every session now and if it lasts for 3-4 weekends, I'll call it fixed...


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